[Netfilter][Watchdog] Use source IP for ban notification WHOIS lookups

[gitterdoneplease]

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What does this PR include?

Short Description

Fail2ban can expand the address that triggered a ban into a configured network range, such as /24. Watchdog currently passes that CIDR range to whois, which may not return useful registration data.

This change stores the triggering IP in a separate Redis hash keyed by the banned network. Watchdog uses that IP for its WHOIS lookup and falls back to the network address when no source mapping is available.

F2B_ACTIVE_BANS remains keyed by the banned network, preserving existing firewall, Web UI, and unban behavior.

This addresses #6843 and implements the separate-key approach discussed in #4759.

Affected Containers

• netfilter-mailcow
• watchdog-mailcow

Did you run tests?

What did you tested?

• Parsed netfilter/main.py with Python's AST parser.
• Validated watchdog.sh with bash -n.
• Validated the Compose configuration with docker-compose config --quiet.
• Checked whitespace and preserved the existing CRLF/no-final-newline format of netfilter/main.py.
• Built local netfilter and watchdog images and deployed them on an active mailcow installation.
• Tested a synthetic /24 ban mapping and then observed multiple naturally generated ban notifications.

What were the final results? (Awaited, got)

Expected:

• Continue displaying and enforcing the configured banned network.
• Query WHOIS using the individual IP that triggered the ban.
• Include complete registration data in watchdog ban notifications.
• Fall back safely when no source-IP mapping exists.

Got:

• Firewall and Redis active-ban entries continued to use the configured network.
• Watchdog notifications continued to identify the banned network.
• Synthetic and naturally generated notifications returned complete WHOIS registration data instead of the CIDR-query error.
• Python, Bash, Compose, and whitespace checks all passed.